Among the many types of conveyor belts in general use today for carrying diverse products along both straight and horizontally curved paths, two types are grid conveyor belts and flat wire conveyor belts. A grid belt is illustrated in U.S. Pat. No. 3,225,898, as well as in FIGS. 1 and 2. Such a belt comprises a plurality of spaced transverse rods slidably interconnected by two rows of connecting links disposed along the inner and outer edges of the rods. The connecting links are disposed in a nested relationship relative to one another with slots in the links slidably receiving the rods. Generally in the prior art, either edge of the belt can suitably collapse when it is necessary for the belt to travel around a horizontal curve.
Grid belts have enjoyed substantial commercial success because of their ability to traverse horizontal curves while still providing an integrated carrying surface capable of supporting a great variety of products. Another desirable feature of such belts is that they can pass freely in vertical curves around relatively small end pulleys. Assuming commonly used link dimension, a typical prior art grid type belt has an inside turn radius in the direction of arrow "A"0 of approximately 2.2 times the width of the belt, as shown in FIG. 3. This is a relatively large horizontal turn radius and is a serious limitation on the use of grid belts due to the resultant inefficient utilization of available floor space.
One of the more significant factors dictating the turn radius for which a conveyor belt can be designed is the internal clearance between the mating links when in the extended position. That is, in order to reduce the turn radius of the belt about a horizontal curve, the links must have an internal clearance great enough to allow the maximum collapse of one link within another link. This requires a significant amount of internal clearance, however, and can cause the belt to run crooked or deviate from a desired straight travel path to a degree unacceptable to the conveyor user. The rods will also not usually be perpendicular to the direction of travel, thus making the conveyor run out-of-square. As shown in FIGS. 1 and 2, two of the commonly used links in a grid conveyor belt have an accumulated lateral clearance, or "shug", of 0.17 and 0.49 inches, respectively, over twelve pitches or links. This distance is noted by the reference "sh" in the drawings. The link dimensions for the belt of FIG. 2 are discussed below with reference to FIG. 3. Thus, the link configurations used to control the shug to an acceptable amount in these belts results in a belt limitation of having a relatively large 2.2 turn ratio.
A small radius conveyor belt, such as that described in U.S. Pat. No. 4,078,655 which is hereby incorporated by reference, overcomes this relatively large turn radius limitation, and thereby further expands the usefulness of grid belts in a variety of conveyor installations such as the spiral conveying systems disclosed in U.S. Pat. Nos. 3,348,659, 3,682,295 and 4,078,655, the contents of which are hereby incorporated by reference. In spiral systems, the belt travels in a spiral direction, and is driven by a cage-like driving drum that is located within the spiral. The small radius conveyor belt of the '655 patent achieves an inside turn radius of approximately 0.9-1.1 times the width of the belt by using tractive central links in combination with inner and outer edge links which are longitudinally expandable and collapsible. The inner links are fully expanded and the outer links are collapsed while moving in a straight path. The inner links then collapse and the outer links expand to allow the belt to traverse a horizontal curve. The small radius belt of the '655 patent is more expensive to manufacture than a typical grid belt, however, due to its more complex structure. In addition, the presence of the central links may be objectionable in certain instances because it limits the quantity of product that can be supported on the belt and it disturbs the even distribution of product on the support area.